The present invention relates to a delay line detector and more particularly to a variable delay line detector which includes a variable delay line which can be programmed to match the characteristics of an incoming signal to enable the detector to be used in applications where the frequency of the incoming ;signal varies, such as in frequency hopped spread spectrum applications.
Delay line detectors are generally known in the art. A conventional delay line detector, generally identify with the reference numeral 20, is illustrated in FIG. 1. The delay line detector 20 includes a power splitter 22, which divides the input signal x(t)=A(t)sin(xcfx89ct) into two paths 24 and 26. The path 24 includes a delay line 28 for delaying the signal in path 24 by d, such that the output signal of the delay line 28 is x(t)=A(t)sin(xcfx89ct+d). The signal in path 26, xxe2x80x2(t)=sin(xcfx89ct), is mixed with the signal x(t) in the output delay line 24 by way of a mixer 30. The output of the mixer 30 is low pass filtered by way of a low pass filter 32 which filters out the xcfx89c terms from the output, resulting in an output signal y(t)=(A(t)/2)*cos(d). The output signal y(t)=(A(t)/2)*cos(d), is a DC signal that represents the phase difference between the signals along paths 24 and 26. As such, the delay line detector functions as a phase detector.
Such delay line detectors are known to be used in various communications and radar applications. For example, such delay line detectors are known to be used as moving target indicators (MTI) as discussed in Communication and Radar Systems, by Nicolaos S. Tzannes, Pentice-Hall Inc., pages 135-137. Such delay line detectors are also known to be used as demodulators for demodulating frequency and phase shift modulated signals in communications systems. In some applications, the delay is configured to be one bit long, causing the delay line detector to function as a data detector or demodulator as generally discussed in Communication Technology Handbook, by Geoff Lewis, Butterworth-Heinemann Publishing Company, copyright 1994, pages 197 and 198 and Electronic Communication Techniques, by Paul H. Young, Charles E. Marrow Publishing company, copyright 1985, pages 477-479, hereby incorporated by reference. In such applications, the delay line is fixed for a specific center frequency, data rate or pulse rate, suitable for the particular application in which the delay line detector is being utilized. As such, the delay line detector cannot be used in applications where the frequency or phase of the input signal varies, such as variable MTI radar applications or as a demodulator for frequency hopped spread spectrum signals. Accordingly, a more complicated and expensive system are known to be used in such applications. Thus, there is a need to provide a relatively less complicated and less expensive method for detecting signals in applications in which the frequency or phase varies.
Briefly, the present invention relates to a variable delay line detector which includes a power splitter, a mixer, and a variable delay line. Various devices are suitable for the variable delay line, such as a non-linear transmission line (NLTL). By providing a variable delay line, the variable delay line detector is adapted to be programmed in real time thus making it suitable in applications where the phase or frequency of the signal varies. As such, the variable delay line detector may be used as a relative inexpensive demodulator in a frequency hopped spread spectrum system.